The proposed studies will employ the principles and methods of operant conditioning. Drug solutions will be substituted for water during a 4-hour, 2-component session. If a drug is to be established as a reinforcer for the rat, the animal must experience the consequences of drug intake. In the proposed studies, intake of substantial quantities of drug solution during the second component (2 hours in duration) will be made highly probable by the use of schedule-induced polydipsia. Schedule-induced polydipsia refers to the drinking of large volumes of liquid which occurs when food-deprived animals are intermittently given small pellets of food and concurrently have access to a liquid. Evidence that a drug has been established as a reinforcer will be provided by the rates of lever pressing maintained during the first component (2 hours in duration). A drug has been established as a reinforcer when its presentation consistently maintains higher lever- press rates than are maintained by water presentation. This procedure, which has been successfully used with ethanol, will be used to determine what drugs (and at what rate and concentration) can be established as oral reinforcers. One major variable to be investigated will be the effect of past drug experience on the rate of development of drugs as oral reinforcers. Past drug experience may consist either of previous drug self- administration (oral or intravenous) or of previous forced drug administration by the experimenter (e.g., making a rat physiologically dependent by morphine injections). Elimination of drug-reinforced behavior will be examined using the technique of reinforcing alternative behavior. Food pellets will be presented to food-deprived rats contingent upon their not pressing the drug lever for a specified period of time; a range of time periods will be investigated. In some cases, additionally the drug-reinforced lever pressing will be extinguished and/or punished while food reinforcement is concurrently provided for alternative behavior.